feat: add support to autodiscover flag

[mtavaresmedeiros]

DRAFT
I create a PR in the Atlantis: runatlantis/atlantis#3279.
In case it is merged I already create the change to the terragrunt-atlantis-config to support this new flag.

Tested:

terragrunt-atlantis-config generate  --output atlantis.yaml --autoplan --auto-discover=false

atlantis.yaml

autodiscover: false
automerge: false
parallel_apply: true
parallel_plan: true
version: 3